Grain boundaries have a significant influence on material properties such as grain growth, creep, diffusion, electrical, optical and last but not least on mechanical properties. Important properties to be considered are e.g. the density of grain boundaries in the material, the chemical composition of the interface and the crystallographic texture, i.e. the grain boundary plane orientations and grain misorientation. A special role is played by the coincidence site lattice (CSL) grain boundaries. CSL grain boundaries are characterized by the multiplicity index X, which is defined as the ratio between the crystal lattice site density of the two grains meeting at the grain boundaries and the density of sites that coincide when superimposing both crystal lattices. For simple structures, it is generally admitted that grain boundaries with low X values have a tendency for low interfacial energy and special properties. Thus, the control of the proportion of special grain boundaries and of the distribution of grain misorientations inferred from the CSL model can be considered to be important to the properties of ceramics and a way to enhance these properties.
In recent years, a scanning electron microscope (SEM)-based technique known as electron backscatter diffraction (EBSD) has emerged and has been used to study grain boundaries in ceramic materials. The EBSD technique is based on automatic analysis of Kikuchi-type diffraction patterns generated by backscattered electrons. A review of the method is provided by: D. J. Prior, A. P. Boyle, F. Brenker, M. C. Cheadle, A. Day, G. Lopez, L. Peruzzo, G. J. Potts, S. M. Reddy, R. Spiess, N. E. Timms, P. W. Trimby, J. Wheeler, L. Zetterström, The application of electron backscatter diffraction and orientation contrast imaging in the SEM to textural problems in rocks, Am. Mineral. 84 (1999) 1741-1759. For each grain of the material to be studied, the crystallographic orientation is determined after indexing of the corresponding diffraction pattern. Available commercial software makes the texture analyses as well as determination of grain boundary character distribution (GBCD) relatively uncomplicated by using EBSD. Application of EBSD to interfaces has allowed the misorientation of grain boundaries to be characterized for large sample populations of boundaries. Typically the misorientation distribution has been linked to the processing conditions of a material. The grain boundary misorientation is achieved via usual orientation parameters such as, the Euler angles, angle/axis pair, or Rodriquez vector. The CSL model is used widely as characterization tool. Over the last decade a research area known as Grain Boundary Engineering (GBE) has emerged. GBE aims to enhance crystallography of the grain boundaries by developing better process conditions and in such way achieve better materials. EBSD has been recently used to characterize hard coatings, for reference see, H. Chien, Z. Ban, P. Prichard, Y. Liu, G. S. Rohrer, “Influence of Microstructure on Residual Thermal Stresses in TiCxN1-x and alpha-Al2O3 Coatings on WC-Co Tool Inserts,” Proceedings of the 17th Plansee Seminar 2009 (Editors: L. S. Sigl, P. Rodhammer, H. Wildner, Plansee Group, Austria) Vol. 2, HM 42/1-11.
U.S. Pat. No. 7,442,433 discloses a tool coating where the upper layer is an alumina layer composed of α-Al2O3 having an average layer thickness in the range of 1 to 15 μm, wherein the α-Al2O3 layer has the distribution ratios of Σ3 to total ΣN+1 in the range of 60 to 80% analyzed by using EBSD (N is any even number equal to or greater than two in consideration of the corundum type hexagonal-packed structure, but if an upper limit of N is 28 from the viewpoint of distribution frequencies, even numbers such as 4, 8, 14, 24 and 26 do not exist). The coating is claimed to exhibit excellent chipping resistance in a high-speed intermittent cutting. The deposition of the α-Al2O3 coatings according to U.S. Pat. No. 7,442,433 is performed from the system H2—CO2—AlCl3-H2S, whereby H2S is applied in the range of 0.1-0.2 vol % and CO2 in the range of 11.2-15 vol %. The ratio CO2/H2S is larger than 75 in all the coatings according to U.S. Pat. No. 7,442,433.